The objective of the proposed research is to define the clonal structure and genetic relationships among pathogenic strains of Escherichia coli that cause enteric disease in humans and animals, with special emphasis on strains that produce cytotoxins (shiga-like toxins or SLTs) that are functionally related to that of Shigella dysenteria. Genetic comparisons, based on the electrophoretic detection of allelic variation at enzyme- encoding genes, will be used to identify pathogenic clones and to establish the overall relatedness of SLT-producing strains to other established groups of pathogenic E. coli, such as strains of enteropathogenic serotypes that have long been associated with outbreaks of infantile diarrhea. A second related aim is to determine the allelic variation in shiga-like toxin genes, through the analysis of DNA polymorphisms using restriction enzymes that recognize four-nucleotide sequences, for the purpose of elucidating the spread of toxins genes into specific chromosomal backgrounds that results in new pathogenic forms. A final aim is to investigate the pattern of geographic spread of a newly described pathogenic clone (serotype 0157:H7) through the analysis of rapidly evolving insertion sequences. The results will be used to draw inferences regarding the role of specific chromosomal backgrounds in enteric disease and the genetic events underlying the origin of new pathogenic forms of E. coli.